1. Field of the Invention
The present invention relates to a method and apparatus for detecting multiple overcurrent thresholds, and more particularly for detecting multiple overcurrent thresholds using a single comparator device.
2. Related Art
In the design of brushless DC motor drive systems, there is often a requirement to sense the dc-link bus current. In brushless DC motor drive systems that are employed for safety-critical applications, such as but not restricted to electro-hydraulic power steering systems, the dc-link bus current is often used in detecting dangerous overcurrent conditions. Some safety-critical systems are required to detect multiple overcurrent thresholds. This enables the detection of multiple overcurrent conditions of various severities. For example, less dangerous overcurrent conditions are detected at lower thresholds, and very dangerous overcurrent conditions are detected at higher thresholds. The system can thereby react differently depending on the severity of an overcurrent condition. An example of circuitry used to measure the dc-link current and detect multiple overcurrent conditions is configured as shown in FIG. 1.
In FIG. 1, the dc-link current idc at the inverter stage 10 is measured using a current sensor 12. For example, this current sensor can be a dc-link shunt resistor, a Hall effect current sensor, etc. An amplifier and filter stage 14 appropriately conditions the measured dc-link current. The output of the amplifier and filter stage is a voltage that represents the dc-link current. This voltage is then presented to multiple comparators 16a, 16b . . . 16n. Each comparator compares the dc-link current to a specific threshold that represents a current level deemed more or less dangerous. In FIG. 1, there are “n” thresholds such that Thres1<Thres2< . . . < Thresn. Thres1 corresponds to the least dangerous overcurrent condition, whereas Thresn corresponds to the most dangerous overcurrent condition. If the dc-link current surpasses one or more of the comparator levels (i.e. Thres1, Thres2, . . . , Thresn), the outputs of the affected comparators switch, indicating to the microcontroller 20 that there is an overcurrent condition.
The inventor has noted two problems with this typical circuit.
A first problem is that using multiple comparators for detecting multiple overcurrent thresholds can be expensive in cost-sensitive applications, such as but not restricted to electro-hydraulic power steering systems. It would therefore be desirable to reduce the number of comparators needed to detect multiple overcurrent thresholds, to a single comparator.
A second problem is that the circuitry used to detect overcurrent conditions must be extremely fast, but its speed is usually reduced by the amplifier and filter stage, which exhibits a propagation delay. This propagation delay is unwanted because it could result in an overcurrent condition damaging the system before the system has time to detect the condition. This propagation delay is unwanted, but is a by-product of the filtering that is used to remove electrical noise. It would therefore be desirable to increase the speed of overcurrent detection.